1. Field of the Invention
The present invention relates to bismaleimide (BMI) resins for use in complex and diverse high performance composite applications. In preferred embodiments, this invention relates to a composition of BMI with improved tack and thermal durability through the incorporation of a curing agent for bismaleimides that unexpectedly is non-crystallizing. The present invention further relates to BMI resin formulations suitable to make prepregs with reduced viscosity for improved prepreg manufacturing, handling properties and handling stability.
2. Description of the Related Art
Fiber reinforced, polymer matrix laminated composite structures (PMCs) are widely used in a number of applications and increasing amounts of composite structures are being used in high performance aerospace applications.
Most composite parts in the aerospace industry use epoxy resins because of epoxy's good combination of mechanical properties, wide service temperature range, and ease of part manufacture. However, some composite applications require higher thermal durability of the finished composite than traditional epoxies can provide.
Epoxy PMCs cannot be used in extreme environments such as high temperature applications, above about 180° C., because they lack adequate thermal durability.
A widely used high service temperature PMC resin currently used is PMR-15, a version of which is sold as CYCOM® 2237 by Cytec Engineered Materials Inc. CYCOM® 2237 has a service temperature of about 288° C. for exposure of less than 100 hours to about 232° C. for exposure of more than 2000 hours. However, since the development of PMR-15 there has been extensive work to find a PMR-15 replacement to overcome its severe limitations restricting its use. The limitations of PMR-15 are micro-cracks and processing difficulty. An additional limitation with PMR-15 is that it contains 4,4′-methylenedianiline, MDA, a health hazard requiring extensive environmental controls.
Where aerospace applications require service temperature beyond the capability of epoxy resins, bismaleimides resins are gaining acceptance because of their epoxy-like processing properties and higher service temperature. Current BMI based resin composite systems offer service temperatures in the range of 149° C. to 232° C. providing excellent mechanical properties such as no micro-cracking and no environmental hazards. For example, Cycom® 5250-4 resin prepreg is offered by Cytec Engineered Materials Inc., as a high temperature primary construction material with a service temperature of about 232° C. for exposure of less than about 100 hours to about 190° C. for exposure of more than 2000 hours.
BMI resins have been modified to improve toughness and manufacturing handling characteristics such as tack through the co-reaction of 2, 2′-diallylbisphenol A (DABA) with substantially aromatic bismaleimides, most specifically bismaleimide incorporating 4,4′-methylenedianaline (MDA-BMI). This process is more fully described in U.S. Pat. No. 4,100,140. However, this system has inadequate thermal durability for 232° C. use as identified by unacceptable weight loss and micro-cracking. Thermal aging at 232° C. demonstrates unacceptable thermal durability for the desired exposure of 2000 hours for many applications.
Additional BMI resin compounds are more fully described in U.S. Pat. No. 5,003,018 and U.S. Pat. No. 5,747,615, which technologies incorporate additional solid, undissolved, BMI to improve tack and drape. These BMI resins give superior mechanical properties, including high service temperature performance, and ease of processing into complex composite parts, but remain inadequate for 232° C. use in excess of 2000 hours as identified by unacceptable weight loss and micro-cracking. In addition, while this art generally discloses that 1,6-hexamethylenediamine bismaleimide (HMDA-BMI) can be incorporated into a BMI resin system, the art does not teach that such an addition can be modified to enhance thermal durability or reduce viscosity to improve tack. Indeed, the art suggests that incorporation of an aliphatic BMI such as HMDA-BMI reduces the Tg and would thus, not be appropriate.
Other improvements in BMI technology were advanced to improve tack, but these advances did not sufficiently reduce the viscosity for processing and the tack remained unacceptable for high temperature applications. The improvement is disclosed as a eutectic blend of the substantially aromatic bismaleimides from MDA-BMI and toluene diamine (TDA-BMI) with an aliphatic bismaleimide derived form 2,2,4-trimethlyhexamethylene diamine (TMH-BMI) in a ratio of about 50/25/15 for MDA-BMI/TDA-BMI/TMH-BMI. These formulations are described more fully in U.S. Pat. No. 4,211,861 and U.S. Pat. No. 4,211,860.
Another limitation of current BMI resin formulations is that additional toughening agents such as thermoplastics are not able to be added to current BMI resin systems because of the inherent high viscosity of current BMI resins systems not allowing the addition of further materials that will further increase the viscosity. The addition of thermoplastics to current BMI resin systems increases the resin viscosity to such a level that the resulting resin viscosity is out of range of practical application.
Current BMI resin systems also have the additional limitation of being unable to fully impregnate carbon fiber prepregs because they contain a high solid BMI content. Current BMI based resin systems contain 35 wt % to 46 wt % undissolved solid BMI, as a slurry in the resin making them notoriously difficult to fully impregnate. As such, to fully impregnate a prepreg incorporating a BMI resin, high processing temperatures are required to reduce the viscosity of the liquid phase component of the resin system and dissolve more solids. Nonetheless, the high solid content of the resin makes manufacturing applications difficult and requires use of low speeds on automatic tape lay-up during part manufacturing. The solid BMI particles in the resin are taught to be necessary to ensure sufficient tack for lay-up, but with more solid particles, the out time is reduced to often less than two days before the tack is reduced to unusable levels.
The present invention solves many of these issues by providing a high temperature composite with increased tack and reduced viscosity to allow for a useable BMI resin based prepreg. This system enhances the mechanical and thermal performance characteristics of the BMI resin in the final composite. Additionally, by reducing the overall viscosity, the present invention allows the incorporation of more total BMI in the resin system to improve mechanical properties as well as the possible incorporation of a thermoplastic.